A general data storage apparatus such as a hard disk drive (HDD) encodes data when storing it on a storage medium and decodes the encoded data when reproducing it. The decoding processing includes error detection/correction to detect an error bit in a bit string of the encoded data and to correct it.
A disk drive realizing a bit-patterned recording (BPR) system in which a magnetic bit pattern is arranged on a disk being a storage medium has been proposed. The disk drive realizing the BPR system stores data on the disk by recording one bit of data in a magnetic cell. Thus, a recorded bit is always 0 or 1, and degradation of recording quality for a bit occurred as a factor of quality of a magnetic transition point is rare.
However, the disk drive still presents the risk of an erroneous bit being recorded around the magnetic transition point because of phase fluctuation or the extent of the recording magnetic field. In this case, since a bit is either 0 or 1 and does not have an intermediate state in the BPR system, if a recording failure occurs, bit inversion is produced instead of an ill-defined bit being recorded. For this reason, for an HDD realizing the BPR system, when a decoding method performing probabilistic estimate calculation based on the likelihood of an error bit is adopted, there are many cases where it is determined that the likelihood is high. That is, although it is desirable that a low likelihood is computed for an error bit, and error correction executed, accurate error detection/correction may be difficult when a decoding method merely performing probabilistic estimate calculation is adopted in the BPR system.